By 1989, Cruddas was the head foreign exchange dealer at the City of London branch of the Jordanian-based Petra Bank. He left the same year to set up his own Business, starting CMC Markets with £10,000 in the bank. CMC Markets is currently valued at between £750 million and £1.2 billion.

Cruddas was resident in Monaco for a period until March 2009, commuting daily from an apartment on Avenue de Spélugues to London City Airport. In an interview in early 2011 he stated that he had: "A £10m apartment in Monaco, a £5m house in Hertfordshire, a house in Antibes, a yacht and a private jet." He plays golf with a low handicap, composes quatrains, and supports Arsenal F.C.

Cruddas was appointed Conservative Party co-treasurer in June 2011 alongside Lord Fink, effectively the party's chief fund raiser, in succession to Billionaire property tycoon David Rowland.

In July 2012, the ConservativeHome blog reported that Cruddas was suing the Sunday Times for libel over its coverage of him. In June 2013, the High Court ruled in his favour and found that the Sunday Times' articles had been defamatory. He was awarded £180,000 in damages on 31 July.

In total, it is believed that Cruddas has donated over £350,000 to the Conservative Party. On 31 July 2013 on the BBC's Newsnight programme he stated that he had donated over £1,000,000. He donated £100,000 in the last quarter of 2010 and £50,000 in the first week of the 2010 general election campaign.

In 2016, Cruddas and his wife Fiona paid £42 million in cash for Balfour House a seven-storey Victorian mansion close to Park Lane in London's Mayfair, formerly owned by the Iranian-born art dealer Nasser Khalili, who lived there for 22 years.

Cruddas supported the Royal Opera House and The Royal Ballet, and after becoming a member of the Chairman’s Circle, in March 2012 was invited to become a Trustee and join the Board of the Royal Opera House.