Thomas resigned as an Assembly Member and his party membership on 25 July 2018, following arrest by police on suspicion of possession of indecent images.

[10] Thomas was first elected as MP for Ceredigion in the 2000 by-election after Dafis stepped down to focus on his role as an Assembly member.

[13] During the 2005 general election he wrote a blog on the Channel 4 News website which shows a near daily account of his campaign trail and his thoughts following his defeat.

In the aftermath of his defeat, he called for Plaid Cymru to reform its leadership arrangements and have a single leader "round which the party coalesces and around which the people of Wales can identify".

In September 2010, Simon Thomas was placed at the top of Plaid Cymru's regional list for Mid and West Wales in the 2011 Welsh Assembly election with the incumbent Nerys Evans instead running for the constituency seat of Carmarthen West and South Pembrokeshire.

[19] When a party leadership election was triggered by the resignation of Ieuan Wyn Jones, Thomas announced his candidacy on 1 December 2011.

[22] Ahead of the 2016 Assembly Election, unsuccessfully sought the Plaid Cymru nomination for Llanelli, but lost out to Helen Mary Jones,[23] he went on to stand in the constituency of Carmarthen West and South Pembrokeshire, coming third as Angela Burns retained it for the Conservatives,[24] he was however returned on the Mid and West Wales List.

As part of the investigation into United Kingdom parliamentary expenses scandal, Thomas was found to have over claimed £349.20 in mortgage interest between 2004 and 2005 under the additional costs allowance (ACA).