Garro-Jones was private secretary to Sir Hamar Greenwood from 1919 to 1922 while Greenwood was firstly Secretary for Overseas Trade and then Chief Secretary for Ireland. Greenwood was a Liberal Minister in the Coalition Government led by David Lloyd George. This close association led Garro-Jones into standing as a candidate for National Liberals at the 1922 General Election. He was selected to contest Bethnal Green North East, where the sitting Liberal member, also a supporter of the Coalition Government, was retiring. However, Garro-Jones's task of holding the seat was made hard when the National Liberals coalition partners, the Unionists, decided to end the coalition and he found a Unionist intervening against him. To make matters worse, he could not count on the support of the local Liberal Association when an opposition Liberal supporter of H. H. Asquith also entered the contest. As a result, he finished bottom of the poll. After the election the divisions in the Liberal ranks between the supporters of Asquith and Lloyd George was heald. Garro-Jones was chosen as Liberal candidate at the 1923 general election for the Unionist seat of Hackney South. No Liberal candidate had fought here at the previous election, so it was not regarded as a particularly good prospect. The Labour candidate won but Garro-Jones was still able to poll more votes than the sitting member who came third. He only had to wait another year for the opportunity to enter parliament came again. He was again chosen as Liberal candidate for Hackney South. However, this time, there was no Unionist candidate and he was able to take the seat from his Labour opponent. His victory was rare in an election which saw a large number of Liberals lose their seats. He stood down at the 1929 election and shortly afterwards joined the Labour Party. He was elected Labour MP for Aberdeen North at the 1935 general election, holding the seat until 1945. Garro-Jones was raised to the peerage as Baron Trefgarne, of Cleddau in the County of Pembroke, on 21 January 1947. In 1954 he assumed by deed poll the surname of Trefgarne in lieu of his patronymic. He was succeeded by his son David, a Conservative government minister.
