[7][6] Thomas-Symonds was called to the bar by Lincoln's Inn in October 2004[7] and developed a practice specialising in chancery and commercial law.

[16][17] He resigned the role of Shadow Employment Minister on 27 June 2016,[18] later supporting Owen Smith in the 2016 Labour Party leadership election.

[22] During the Brexit debate, Thomas-Symonds regularly clashed with the then Attorney General, Geoffrey Cox, across the despatch box.

[citation needed] In a debate on 3 December 2018, after The Sunday Times newspaper reported leaked excerpts of a recent letter sent by the Attorney General to Cabinet Ministers detailing legal advice on Prime Minister Theresa May's Brexit deal, Thomas-Symonds accused Cox of hiding his full legal advice on May's Brexit deal “for fear of the political consequences", stating that the government was "playing for time, hoping that the contempt proceedings take longer than the timetable for the meaningful vote".

On the same day, leaked minutes of a conference call between cabinet ministers, including Cox, which took place after the prorogation had been approved by the Queen, detailed that Cox briefly told the cabinet at that time that in his view the prorogation was lawful and constitutional and that any accusations of unlawfulness "were motivated by political considerations".

[30] He has served as Chair of four All-Party Parliamentary Groups: Off-Patent Drugs; Industrial Heritage; Legal & Constitutional Affairs; and Archives & History.

[33] As a result of promises secured by Thomas-Symonds, the British National Formulary has started to include off-label drugs in its new indication where there is sufficient evidence.

[35] In April 2020, Thomas-Symonds was appointed as Shadow Home Secretary by Sir Keir Starmer, succeeding Diane Abbott.

Amongst his first acts in post, Thomas-Symonds called on the Home Secretary, Rt Hon Priti Patel MP, to provide additional funding for organisations tackling domestic abuse.

[37] After the murder of George Floyd on 25 May 2020, Thomas-Symonds urged the Prime Minister to "show leadership" in addressing structural racism and inequality.

"[43] During the Covid pandemic, Thomas-Symonds argued for better protections at the border, including comprehensive hotel quarantining for arrivals in the UK.

[45] In March 2021, Sarah Everard disappeared after leaving a friend's house near Clapham Common to walk home.

In response to the tragedy, and the vigil held on Clapham Common, Thomas-Symonds called for action to end violence against women and girls, putting forward a 10-point plan alongside Labour's Shadow Minister for Domestic Violence and Safeguarding, Jess Phillips MP.

[53] In his first full interview, Nick Thomas-Symonds said that the British public had been promised a US-UK trade deal and that one should be delivered.

"[55] On 3 March 2022, following the Russian invasion of Ukraine, Nick Thomas-Symonds called on the UK to place a total ban on exporting luxury goods to Russia to target the "Mayfair lifestyle" enjoyed by President Putin and his inner circle.