[2] After completing his Diploma in Legal Practice in 2008, Masterton obtained a traineeship with the law firm McGrigors, working in their Glasgow and Belfast offices.

In 2010, Masterton became a newly qualified solicitor specialising in Pensions[3] and long-term savings, continuing to work with McGrigors, latterly Pinsent Masons, for 9 years, until his election.

[6] He was an active member of a number of other APPGs including those on Surrogacy, Pensions, Equitable Life, Post Offices and Single Parents.

[8] In April 2019, Masterton led a debate highlighting the impact of the tapered annual allowance on senior clinical staff who were members of the NHS Pension Fund.

[9] Masterton worked closely with the BMA and NHS Employers, successfully lobbying the Department for Health and Social Care, and the Treasury, to make changes to the Scheme and launch a review into the operation of the annual allowance taper.

[12] In November 2017, Masterton was listed by The Daily Telegraph as one of fifteen Tory "Brexit mutineers",[13] within a week he had received an anonymous Christmas card describing him as a "traitor".