Alexander F.I. Forbes was educated in Scotland. His father was an amateur astronomer who built his own telescope and lived on the estate of David Gill*, himself at that time an enthusiastic amateur astronomer. Forbes senior shared his interest in astronomy with his son. The latter came to the Cape Colony in 1896. He resided in Woodstock and worked as a builder until 1907, when he returned to Scotland to study architecture in Aberdeen. After qualifying in 1909 he became a member of the (British) Institute of Architects (MIA) and practised as an architect in Cape Town until 1932. During these years he lived in Rosebank and soon after his arrival built a 200 mm reflecting telescope and a small observatory at his house, "Craigie Brae" in Liesbeek Road.
Forbes was a member of the Cape Astronomical Association (founded in 1912) and was serving on its council as honorary treasurer in July 1922, when it amalagamated with the Johannesburg Astronomical Association to form the Astronomical Society of South (later Southern) Africa. He was a an alternative member of council of the new society from 1923 to 1932, serving as honorary treasurer of the society, and as committee member and honorary treasurer of its Cape Centre. During 1926-1930 he was also the society's librarian. From 1937 he served as a member of council and was elected president of the society for 1942-1943.
Forbes was a regular lecturer on astronomical topics. For example, he read a paper on "Reflecting telescopes, with practical directions for grinding and figuring the mirror" before the members of the Cape Astronomical Association during 1921/22, and addressed the Natal Astronomical Association on "Satalites and their movements" in 1927. However, his main contribution to the society was as director of the Comet Section from 1929 to 1945 and director of the Zodiacal Light Section from 1934. He was a systematic and persistent comet hunter and after an eight month search obtained his first success when he rediscovered Comet Pons-Coggia-Winnecke on 19 November 1928. For this achievement he received the Donohue medal of the Astronomical Society of the Pacific. On 1 August1929 he discovered comet 1929c (Forbes), which has a relatively short period of just over 6 years. His next discovery, 1930e (Forbes), has a parabolic orbit and was identified on 29 May 1930. His last find, 1932n (Dodwell-Forbes), was discovered at Hermanus on 15 December 1932 and has a long period.
Forbes contributed some articles on astronomical instruments to the Journal of the Astronomical Society of Southern Africa: 'An articulated tripod stand for a telescope' (1926), 'Observatory dome' (1938), and 'Blink finder for telescope' (1938). He also contributed several articles on his comet work to the Monthly Notes of the Astronomical Society of Southern Africa (MNASSA): 'Comet Van Gent' (1941), 'Comets (Presidential address)' (1943), and 'South Africa's place in cometary discovery' (1947). His interest in the 5 m Hale reflector in the USA led him to build a scale model of this instrument, which was later presented to the South African Museum, Cape Town.
In 1932 Forbes retired to Hermanus, where he built another observatory at his house "Blairythan". He also continued with architectural work there for the next 20 years, though at a slower pace. In 1956 he returned to Cape Town, where he lived with his niece for the remaining few years of his life. As a modest and unassuming person he did not search publicity, but was an able person with many and varied interests and hobbies, and a thirst for knowledge.