In 1967 he was appointed coach of North American Soccer League side Baltimore Bays. After two years Jago was named coach of the United States national team but ended this role after suffering two defeats in his only games in charge. In 1970 Jago joined Queens Park Rangers as a coach and became manager in January 1971. It was under his guidance that the basis of the QPR team which in 1975–76 would come within a point of the League title was assembled. He signed pivotal players such as Stan Bowles, Don Givens, Dave Thomas and Frank McLintock and led the club to promotion to the First Division in 1972/3. Jago left the club in October 1974 and was later appointed manager of Millwall where he remained until 1977. In 1976 Jago took Millwall from the Third to the Second Division.
He returned to North America to coach NASL side Tampa Bay Rowdies between 1978 and 1982. He saw great success especially early on in Tampa, losing in the outdoor final in both 1978 and 1979 and winning an indoor championship in his second year. Jago would return the Rowdies to the indoor final in the 1981-82 season, but came up short on his second try. He took the Rowdies to the outdoor post-season in four of his five years in Tampa, before resigning in the latter part of a disappointing 1982 campaign.