In 1912, he obtained Ph.D. from Germany and came back to India, and from 1913 he started his legal profession in Patna under the able guidance of Mazharul Haq.

[2][3] Throughout his career he insisted on communal harmony, played significant role in the Congress- League Pact of Lucknow in 1916.

Served with the Home Rule League, AICC and gave up his legal practice to participate in the Khilafat Movement.

In 1923, he was elected Deputy General Secretary of the AICC along with Jawaharlal Nehru which resulted in close friendship between the two leaders.

The Sri Krishna Sinha led cabinet in Bihar made Syed Mahmud Minister for Education, Development and Planning in 1937.

His emphasis was on providing primary education to largest possible number of people, worked for revision of curricula, appointed Urdu teachers in the Patna University.

Mahmud was one of the members of the Congress Working Committee that endorsed the 1942 Quit India movement, calling for an immediate end to British rule.

From August 1942, Mahmud and all of the other top leaders of the Quit India movement were imprisoned, mostly in Ahmednagar Fort.

In 1949 he suggested Nehru to enter into a particular military pact with Pakistan in order to safeguard the nation from China which could not materialize.

From 7 December 1954 to 17 April 1957 he was the Union Minister of State for External Affairs but resigned due to eye troubles.

He played remarkable roles in India's useful diplomatic relations with the Gulf countries, Iran and Egypt.