He was the third child of Ishwar Devi and the pioneer Indian meteorologist and scientist Ruchi Ram Sahni who lived in Lahore.

The family came from Dera Ismail Khan and they frequently made visits to Bhera which was close to the Salt Range and Khewra's geology may have interested Birbal at a young age.

Birbal was also influenced into science by his grandfather who owned a banking business at Dera Ismail Khan and conducted amateur research in chemistry.

[8][9][10] During his stint in England, Sahni joined Professor Seward to work on a Revision of Indian Gondwana plants (1920, Palaeontologica Indica).

In 1919 he briefly worked in Munich with the German plant morphologist Karl Ritter von Goebel.

Sahni maintained close relations with researchers around the globe, being a friend of Chester A. Arnold, noted American paleobotanist who later served his year in residence from 1958–1959 at the institute.

On 3 April 1949 the Prime Minister of India Jawaharlal Nehru laid the foundation stone of the new building of the institute.

[13] Sahni work on living plants species including Nephrolepsis, Niphobolus, Taxus, Psilotum, Tmesipteris and Acmopyle examining evolutionary trends and geographical distributions.

Sahni was among the first to suggest a separate order, the Taxales, within the conifers to contain the genera Taxus, Torreya and Cephalotaxus.

[16] Sahni identified Torreyites, a close relative of Torreya, which extended the range of the Taxales into Gondwanaland.

He suggested that the lower Narmada area around Nagpur and Chhindwara was coastal on the basis of fossils that showed a similarity to estuarine palms of the genus Nipa.

[17] Based on the ecology of plants and the altitude of the fossil finds, he also attempted to estimate rates of uplift of the Himalayas.

He was elected a Fellow of the Royal Society of London (FRS) in 1936, the highest British scientific honour, awarded for the first time to an Indian botanist.