The Belgian was punished for violations committed while he was working with Rabobank between 1996 and 2009, before he joined Team Sky.
The U.S. Anti-Doping Agency announced that a three-member American Arbitration Association panel found that Leinders "possessed, trafficked, and administered banned performance-enhancing substances and methods without any legitimate medical need ... to athletes under his care, and was complicit in other anti-doping rule violations."
USADA, which worked on the case with anti-doping authorities in Denmark and the Netherlands, said Leinders was involved in doping with EPO, blood transfusions, testosterone, insulin, DHEA, corticosteroids and other substances.
"It shocks the conscious that a board member and team doctor would abuse his trusted position by overseeing and participating in this type of dangerous and fraudulent activity," USADA CEO Travis Tygart said.
The case was based on evidence discovered by USADA in 2012 during its investigation of doping in cycling that led to the demise of Lance Armstrong.
USADA said Leinders was sanctioned after a hearing that included presentation of "eyewitness testimony, corroborating documentation and an expert analysis of abnormal blood values of cyclists."
Leinders was hired by Sky in 2011 and 2012 on a freelance basis but has left the team.
"The Arbitration Panel agreed with the charges that the alleged doping offences involved aggravating circumstances, which justify a lifetime period of ineligibility," USADA said in a statement.
"As the chief team physician and a member of the board of directors for the Rabobank cycling team, Dr. Leinders occupied even higher positions of trust and responsibility" than other support personnel, trainers, coaches and team physicians who have been sanctioned for doping violations, USADA said.
In 2007, Rabobank removed Danish rider Michael Rasmussen from the Tour de France while he was leading the race after he repeatedly declined to explain why he missed three doping tests ahead of the event. He was subsequently given a two-year ban and later admitted that he took banned drugs for more than a decade.
Rasmussen's co-operation and testimony in the investigation of Leinders were "integral to the outcome," said the head of the Danish anti-doping agency, Lone Hansen.
Leinders is now banned from training of advising athletes and can't take part in "any event sanctioned by USA Cycling, the International Cycling Union (UCI), or any other WADA Code signatory."
After ending his team's collaboration with Leinders, team Sky boss Dave Brailsford said that hiring him was a mistake and that he would have never employed him if he had knew about his doping past.