It's in his stewardship major relief operations were conducted in various parts of India, Nepal and Bangladesh during natural calamities.
The 98-year-old monk, who had spent several years at Shyamla Tal in the Himalayas, had been undergoing treatment for age-related ailments since February 2015.
The monk, President Maharaj to tens of thousands of devotees since December 2007, had been undergoing treatment for age-related ailments at Ramakrishna Mission Seva Pratishthan since February 2015. His condition worsened on Saturday and he was put on ventilator.
As news of his death spread, hundreds of followers thronged the hospital premises to pay their final respects.
A statement released by the Ramakrishna Math and Ramakrishna Mission, Belur Math, said, "Despite the best medical attention, his condition deteriorated over the last few days and he passed away around 5.30 pm today at Ramakrishna Mission Seva Pratishthan hospital."
The mortal remains reached Belur Math, the global headquarters of Ramakrishna Math and Mission, at 8.20pm and will be kept in the cultural hall for devotees to pay their last respects.
The cremation is scheduled for 9.30pm on Monday.
"The gates of Belur Math will remain open all through the night of the 18th and through the 19th till the last rites are completed," the Math and Mission said in a statement.
Prime Minister Narendra Modi, who had met the ailing Swami on April 17 last year, condoled the seer's death, terming it as a "personal loss".
As a young boy, Modi had visited the Belur Math to join the Order, but his request was turned down and he was told that his calling was elsewhere.
Later, he got spiritual guidance from Swami Atmasthananda in Rajkot, Gujarat and considered him as his 'guru'.
West Bengal Governor Keshari Nath Tripathi and Chief Minister Mamata Banerjee also condoled the demise of the monk.
Banerjee had paid a visit to the ailing monk earlier in the afternoon.
Who was Swami Atmasthananda
Swami Atmasthananda was born on Buddha Purnima in May 1919 at Sabajpur near Dhaka. He had received mantra diksha (spiritual initiation) from Swami Vijnanananda (a monastic disciple of Sri Ramakrishna) in 1938 and joined the Ramakrishna order at Belur Math on January 3, 1941, at the age of 21.
In 1945, Swami Virajananda, then president of the Ramakrishna Math and Mission, gave the youth the brahmacharya vows. He got the sannyasa vows and the name Atmasthananda in 1949.
After serving the order at Belur Math and the branches in Deoghar (Ramakrishna Mission Vidyapith) and Mayavati (Advaita Ashrama), Swami Atmasthananda got an opportunity to serve Swami Virajananda for a long time. He spent several years in the then president's company in the solitude of Shyamla Tal in the Himalayas.
In 1952, Swami Atmasthananda joined Ramakrishna Mission TB Sanatorium as assistant secretary. He worked hard to expand the centre's activities.
The monk was sent to the Rangoon Sevashrama as its secretary in 1958. Under his guidance, the Sevashrama hospital emerged as one of the best hospitals of Burma.
Swami Atmasthananda returned to India in 1965 after Burma's military rulers took over the hospital.
The swami joined the Rajkot branch of the Math and Mission as its head in 1966. The beautiful temple of Sri Ramakrishna at the Rajkot Ashrama was built on his initiative.
Days after the swami had moved to Rajkot, a 16-year-old boy named Narendra Modi met him and expressed his desire to join the order.
Modi, now the Prime Minister, tweeted several times in the evening expressing grief at the passing away of the President Maharaj.
"The demise of Swami Atmasthananda ji is a personal loss for me. I lived with him during a very important period of my life," was Modi's first tweet.
It was soon followed by another: "Swami Atmasthananda ji was blessed with immense knowledge & wisdom. Generations will remember his exemplary personality."
Two other tweets of the Prime Minister read:
"Whenever I would visit Kolkata, I would always make it a point to seek the blessings of Swami Atmasthananda ji."
"As President of Ramakrishna Mission set up by Swami Vivekananda, Swami Atmasthananda ji worked tirelessly & spread its influence globally."
During a visit to Belur Math on April 9, 2013, Modi, then Gujarat chief minister, had said: "I had the fortune of spending time with Swami Atmasthanandaji during my childhood. I have received a lot of affection from him. He has been by my side and supported me at every crucial juncture of my life."
Modi's reverence for the monk - the two would usually converse in Gujarati - was evident when he carried a flower bearing Swami Atmasthananda's blessings in his jacket while taking oath as Prime Minister on May 26, 2014. The monk had also sent him a letter, which reportedly addressed Modi as "Narendrabhai".
When 16-year-old Modi visited the Rajkot centre in 1966 and told Swami Atmasthananda that he wanted to become a monk, the swami tried to dissuade him.
On the teenager's insistence, the monk sent him to meet the then president of the order, Swami Madhavananda, in Belur Math. The president told Modi that his calling lay elsewhere.
Chief minister Mamata Banerjee had visited the ailing monk hours before his death. "I learnt he is in a critical state and doctors have put him on ventilator," she had said at the hospital.
She later tweeted: "Saddened that Rev. Swami Atmasthanandaji, President, Ramakrishna Math & Mission passed away today at Seva Pratishthan."
"A life of outstanding social and religious service," she added.
Swami Atmasthananda was elected a trustee of the Ramakrishna Math and a member of the governing body of the Ramakrishna Mission in 1973. In 1975, he was appointed an assistant secretary of the twin organisations.
His tenure as relief secretary saw the Math and Mission conduct massive relief and rehabilitation operations in India, Nepal and Bangladesh.
Swami Atmasthananda became general secretary - the chief executive - of the Math and Mission in 1992 and remained in the post till he became a vice-president in 1997.
Swami Atmasthananda was elected president of the order on December 3, 2007, following the demise of Swami Gahanananda, the 14th head of the Math and Mission.
As a vice-president of the Math and Mission, Swami Atmasthananda went to various parts of the country and the world and visited many branches of the order and some unaffiliated centres.
"In 1998, he visited various places in the US, Canada, Japan and Singapore. He also went to Malaysia, Fiji, Sri Lanka and Bangladesh at different times. In all those places, he spread the message of Sri Ramakrishna, Sri Sarada Devi, Swami Vivekananda and Vedanta and also gave mantra diksha to many seekers," the Math and Mission said in a statement.
The cremation will take place at Belur Math tomorrow at about 9.30 pm and the gates of Belur Math would remain open tonight and tomorrow till the last rites were completed, it stated.
Prime Minister Narendra Modi, who had met the ailing Swami on April 17 last year, condoled the seers death, terming it as a "personal loss".
As a young boy, Modi had visited the Belur Math to join the Order, but his request was turned down and he was told that his calling was elsewhere.
Later, he got spiritual guidance from Swami Atmasthananda in Rajkot, Gujarat and considered him as his guru.
West Bengal Governor Keshari Nath Tripathi and Chief Minister Mamata Banerjee also condoled the demise of the monk.
Banerjee had paid a visit to the ailing monk earlier in the afternoon.
Atmasthanandaji, born in May 10, 1919, at Sabajpur, near Dhaka, now in Bangladesh, had received "mantra diksha" from Swami Vijnananandaji Maharaj (a monastic disciple of Ramakrishna) in 1938 and joined the Ramakrishna Order at Belur Math in January, 1941 at the age of 22.
In 1945, Swami Virajanandaji Maharaj, the sixth president of the Order, gave him brahmacharya vows, and in 1949, sanyasa vows and the name Swami Atmasthananda.
After serving the Order at Belur Math and the branches at Deoghar (Vidyapith) and Mayavati (Advaita Ashrama), Atmasthanandaji got the opportunity to serve Swami Virajanandaji Maharaj, the then president of the Order, for a long time.
He spent several years in the company of Virajanandaji Maharaj at the Shyamla Tal in the Himalayas. In 1952, he was posted to Ranchi TB Sanatorium branch as an assistant secretary.
In 1958, he was sent to Rangoon (Yangon) in Burma (now Myanmar) Sevashrama as its secretary.
When military rulers took over the Rangoon Sevashrama, he returned to India in 1965 and was posted at the Rajkot branch as its head in 1966.
He was elected a trustee of the Ramakrishna Math and member of the governing body of the Ramakrishna Mission in 1973.
In 1975, he was appointed an assistant general secretary of the twin organisations. Under his leadership as relief secretary, the math and mission conducted massive relief and rehabilitation operations in various parts of India, Nepal and Bangladesh during natural calamities.
He became the general secretary of the math and mission in 1992 and remained at that post for five years till 1997 when he became a vice-president of the Order.
He was elected president of the Ramakrishna Math and Ramakrishna Mission in December 2007.
Swami Atmasthanandaji had travelled extensively all over the country and abroad and visited several branches of the Order and some unaffiliated centres spreading the messages of Ramakrishna, Sarada Devi, Swami Vivekananda and Vedanta, besides giving mantra diksha to disciples.
He was one of the key persons to organise the Pallimangal (integrated rural development) activities in Kamarpukur and Jayrambati, a youth training centre at Saradapitha (Belur) and many other social service programmes.
Governor Tripathi conveyed his heartfelt condolences to Atmasthanandajis numerous devotees whose "loss cannot be compensated".
"May his soul rest in peace in his heavenly abode," Tripathi said in a statement released by the Raj Bhawan tonight.
Banerjee described his passing away as "an irreparable loss to the mankind".
"Saddened that Rev. Swami Atmasthanandaji, President, Ramakrishna Math & Mission passed away today at Seva Pratishthan #Kolkata," she tweeted.
"I paid my last visit to him this afternoon. A life of outstanding social & religious service. This is an irreparable loss to mankind," the chief minister said.
(With inputs from The Telegraph Calcutta and PTI)