In 1983 Peter Hardy, Labour MP for Rother Valley, decided to switch constituencies to fight the equally safe neighbouring new seat of Wentworth. With the NUM backing Barron secured the nomination and was duly elected as the Labour MP for Rother Valley at the 1983 general election.
In 1985 Barron was made a Parliamentary Private Secretary to the Leader of the Opposition Neil Kinnock, a position he held until the 1988. Kinnock gave Barron a frontbench job in 1988 as an opposition spokesman on Energy he lost this position when John Smith took over the leadership and he refused another front bench position. Barron was returned to the front bench nine months later as a spokesman on Employment by the new leader John Smith, and after Smith's death Tony Blair moved Barron to speak on Health matters.
Barron was a leading figure in the campaign to rewrite Clause IV under the new leadership of Tony Blair and it came as a surprise that there was no job in government for him after the victorious 1997 general election. He served for eight years on the senior Intelligence and Security Committee and was made a Privy Councillor in 2001. He was made Chairman of the influential Health Select Committee after the 2005 general election.
Barron has been a Member of the General Medical Council since 1999, and is passionately anti smoking.
Following the MP's expenses row, Barron placed his expenses claims cover sheets in the window of his constituency office in Laughton Road, Dinnington. In the review of Past Additional Cost Allowance by Sir Thomas Legg QC Barron was found to have no issues and was not required to repay any monies. In May 2010, Barron was returned to Westminster as the MP for Rother Valley with a severely reduced majority. In July 2010, he was appointed chair of the parliamentary Standards and Privileges Committee
Barron was knighted in the 2014 New Year Honours for political and public service.