Weir’s first appearance for Scotland was on 10 November 1990 against Argentina at Murrayfield Stadium. A mainstay of the team throughout the 1990s, he was a fan favourite of the Murrayfield crowd. In 1997 he was the first recipient of the Famous Grouse Scotland Player of the Five Nations Award.

He later moved to England in 1995 to join the Newcastle Falcons and was part of the Premiership winning side of 1997–98.

A lineout specialist, he was selected as part of the British and Irish Lions tour of South Africa in 1997. Whilst on the tour he suffered a serious knee injury, as a result of foul play, while playing against Mpumalanga Province.

His time in the national side declined in later years as the next generation of locks emerged, with the likes of Stuart Grimes and eventual Scotland cap record holder Scott Murray coming into the team. His final appearance was in the Six Nations Championship match against France at Murrayfield, on 4 March 2000. He won 61 Scottish caps, scoring 19 points from four tries (his first scoring four points under the old scoring system).

He moved back to Scotland to join the newly reformed Borders team in 2002 where he remained until his retirement from professional rugby. He finished his playing career together with Gary Armstrong at the Border Reivers in 2004.

In June 2017, to promote Global MND Awareness Day, Weir announced that he had been diagnosed with motor neurone disease (MND). In August, he spoke about plans to set up a foundation named ‘My Name’5 Doddie’ in order to “raise funds for research into a cure for MND and to provide grants to people living with the condition”.

A new tartan was designed by Doddie in collaboration with Berwickshire-based clothes firm ScotlandShop, in a bid to raise cash for his motor neurone disease research charity. The tartan features colours from the teams he played for: black and yellow of Melrose, blue and white of Scotland, and black of the former and white of the latter are also intended as a reference to his seven years with Newcastle Falcons.My Name’s Doddie: The Autobiography was published on 25 October 2018 through Black & White Publishing.

On 31 October 2018, Weir appeared on BBC One’s The One Show, where he stated that his charity had now raised over £1 million.

He was appointed an Officer of the Order of the British Empire (OBE) in the 2019 New Year Honours for services to rugby, to motor neurone disease research and to the community in the Scottish Borders.

In December 2019, Weir was announced as the recipient of the Helen Rollason Award, which is presented every year during the annual BBC Sports Personality of the Year show.