Nigel Mansell the only driver to hold both the Formula One and Indy Car Championship at the same time

One more.  Jacques Villeneuve won the F1 and Indycar championships. 

All talented race drivers. Mansell is my favourite of the three despite his fiery personality.

His spectacular crash at Phoenix ( not his fault) before returning to Formula 1 after Senna’s death was a likely factor in his final retirement from racing.

@david-green Actually the crash at Phoenix was Mansell's fault, Jim McGee, Mansell's chief engineer on his Newman/Haas Lola talked about the Phoenix crash, he said he talked to Mansell about how to drive the oval but Mansell told him that Senna, Prost and himself has learned how to maneuver through corners much faster due to the blown exhaust that was used as it would suck the car down onto the track. The problem was you have to lift at Phoenix and when you lift you lose all that down force from the blown exhaust. McGee said that Mansell changed how he drove after that crash on the ovals. 

Nigel Mansell himself has also said that he had to adjust how he drove on the ovals after that crash. Mansell in interviews has said that he would follow Emerson Fittipaldi and Paul Tracy on the ovals as they would overtake Mansell on the starts of the races and Mansell said he would follow them learning the lines how to drive the track during the race. Mansell was a quick study as evidence by 4 of his five wins came on the ovals. If not for the yellow flag with just a few laps to go at the 1993 Indy 500 Mansell would have won that race as well. 

The 1993 Penske chassis was better than the Lola on Road Courses while the Lola was even or just a little better on the ovals. The 1994 Lola had fallen behind the Penske on both road courses and ovals. Bernie Ecclestone worried about the world wide attention Mansell brought to Indy Car bought out Nigel Mansell's contract to get him out of Indy Car. Nigel Mansell had a contract with Newman/Haas through the 1997 season. 

Mansell raced the last four races of the 1994 Formula One season with Williams and he won the final race of the season as well as taking the pole. But once again Frank Williams refused to pay and Mansell took an offer from McLaren but the McLaren chassis was not very good and was built to narrow for Mansell to properly fit, Mansell missed the first two races of the 1995 F1 season because of that while McLaren built new wider chassis' but the car still was not very good and had bad understeer so Mansell retired for good after his second race. McLaren never really wanted Mansell, I think that was more of Bernie Ecclestone's doing. 

@john3976 

Thank you John. That was comprehensive thoughtful reply. I learned a bit, corrected a bit in my mind and re-remembered a few things.