Now he is operating from the more humble environs of a trading estate in downtown Kilburn, west London. But he could still be in the money.
This week sees the beginning of share dealings in My Kinda Town, the Chicago Rib Shack and Henry J Bean restaurant business founded by the larger-than-life American, Bob Payton. Johnson has pounds 125,000 in the project.
The son of the right-wing polemicist Paul Johnson, thirty- something Luke has teamed up with an old buddy for the venture. His partner is Hugh Osmond, whom he met when they were both medical students at Oxford University. With City backing, the pair are paying pounds 15.7m to buy the business from institutional investors and Payton, who now spends most of his time in the luxurious surrounds of Stapleford Park, his country house hotel in Leicestershire.
Johnson junior's career has progressed better than he might have hoped since his Kleinwort days. 'I wasn't really much good as an analyst,' he now admits. 'And it was fairly obvious that I didn't fit into the culture at Kleinworts. I remember telling the Wall Street Journal that the Sid campaign for British Gas was rubbish before realising that Kleinwort was one of the sponsors. The paper quoted me on the front page.'
That wasn't his only bit of bad luck. The launch of his book coincided with the stock market crash - and the highly paid jobs he had referred to grew a bit harder to come by.
Johnson left to sample life as an entrepreneur, and he's had a good run. As a double act, Johnson and Osmond have have alighted upon Pizza Express, bridalwear company Cupid, Utility Cable and a group called Crabtree.
Johnson, who says he can get by on his current income of pounds 50,000, describes himself as an opportunist. Others might call him a gambler, though he and his partner are well placed to judge. They once co-wrote a book called Betting to Win.