Born 11 February 1934, Weir attended the Royal Academy of Dramatic Art in the 1950s, and began writing scripts for television in the 1960s. He was the writer of many popular 1960s and 70s series, notably The Plane Makers (1963-4) and The Troubleshooters, (1966—69; known in the USA as Mogul), as well as The Lotus Eaters (1972).

Weir also wrote occasional scripts for many other British TV series, including Danger Man (1964), Crown Court, A Family at War, 1999, The Onedin Line and others.

Weir’s most popular television work was probably his English adaptations of the cult BBC TV series The Water Margin (c. 1977), based on the traditional Chinese story Water Margin and Monkey! (1978—80). These two series were produced in Japan and China using local actors and crew and dubbed into English using British voice over artists. The original English scripts were created and written by Weir without available translations, using only brief plot synopses. Monkey! still has a strong fan following today and is described as a 'cult' classic.

Weir also wrote the novel The Water Margin, based on the TV series..

The only known interview with Weir was published in The Radio Times 10—17 November 1979, at the start of the second series of Monkey! In this interview he revealed his interest in Buddhism and Eastern culture and religion, which has developed throughout the rest of his life.

Weir wrote scripts for six episodes of Doctor Who that were considered too expensive for contemporary budgets and were never made, although they were due to be part of series fifteen starring Tom Baker in 1978. The episodes, titled The Killers of the Dark, introduced a race of cat people living on the Time Lords' home planet of Gallifrey. It is not known whether Weir's scripts still exist, or whether the BBC has any plans to make the episodes in future.

Weir is retired and lives a reclusive life in a small village in rural England.