While still a student, he volunteered with the Child Poverty Action Group and worked with then-Chief Executive, Kate Green, who became a Labour MP. Before becoming an MP himself, Pennycook worked for a number of charitable and voluntary organisations including the Fair Pay Network and the Resolution Foundation where he led on issues relating to welfare reform, low pay and working poverty. He also worked for a while in Parliament as an assistant to Labour MP Karen Buck.
Pennycook was a Labour councillor for Greenwich West from 2010 to 2015, resigning in March 2015 just before the general election.[8] He also served as a trustee of Greenwich Housing Rights and was a school governor at James Wolfe Primary School in West Greenwich. He has written multiple articles for The Guardian about the need for a living wage in the UK and has served on the Living Wage Foundation's advisory board.[9]
At the 2015 general election, Pennycook was elected as MP for Greenwich and Woolwich with a majority of 11,946 votes and a 52.2% share of the vote on a turnout of 63.7%.[11][12] Pennycook gave his maiden speech in the House of Commons during a debate on the economy on 4 June 2015.[13]
In July 2015, Pennycook became a member of the Energy and Climate Change Select Committee. He served as Parliamentary Private Secretary to Shadow Minister of State for Housing, John Healey MP from 2015, resigning from the position in June 2016.
Pennycook campaigned in favour of a "Remain" vote for the 2016 referendum on EU membership[15] and his Greenwich and Woolwich constituency voted 64% to remain.[16] After the referendum results were announced, Pennycook was appointed one of the Shadow Ministers for Brexit in October 2016,[17] and, in accordance with the Labour Party whip, voted for the Bill to trigger Article 50.[18]
At the snap 2017 general election, Pennycook was re-elected as MP for Greenwich and Woolwich with an increased vote share of 64.4% and an increased majority of 20,714.[19][20]
In October 2018, Pennycook expressed concerns about newly qualified teachers leaving the profession. He stated in an interview that "The crisis in teacher retention in London did not begin the day before yesterday, yet this Tory government still has no coherent plan to address the problem and no appetite to get to grips with the underlying drivers – workload, stagnant pay, rising living costs and a lack of genuinely affordable housing to rent and buy – that lie behind this worrying trend."[21]
In September 2019, he resigned as shadow Brexit minister in order to campaign actively in favour of holding a second referendum and unequivocally for the UK to stay in the EU.[22]
At the 2019 general election, Pennycook was again re-elected, seeing his share of the vote decrease to 56.8% and his majority reduced to 18,464.[23][24][25]
In May 2021, Pennycook announced his opposition to a 1,500-home development project in his constituency over concerns about the height of its planned high rises.[28]
In March 2022, Pennycock argued that housing supply is not a "panacea for affordability".[29]
At the 2024 general election, Pennycook was again re-elected, with an increased vote share of 56.2% and a decreased majority of 18,366.[30]