Starting at Wrexham at the age of 14, Niedzwiecki stayed with the club until the summer of 1983, when he was signed for Chelsea by former Wrexham manager, John Neal. He quickly won a regular place in the Chelsea starting line-up and was impressive as the side romped to the Second Division title in his first season. He also earned two caps for Wales during his playing career.
Niedzwiecki was forced to retire aged 28, after failing to recover from numerous injury problems. He later went on to become a coach at Chelsea, leaving the club in November 2000 after the arrival of Claudio Ranieri. He joined Arsenal the following month, succeeding the late George Armstrong as reserve team coach. He also worked part-time as coach with Wales, with Mark Hughes as manager for the Welsh, during this time. In September 2004, he joined Blackburn Rovers, under Mark Hughes, as first-team coach. Four years later, when Hughes left for Manchester City, he was among the several backroom staff from Blackburn to follow him in making the switch.
However he became unemployed on 19 December 2009 when Hughes and his complete backroom staff were relieved of their duties at the club. In 2010 he once again linked up with Hughes at Fulham as part of the new Fulham manager's coaching staff. He left in the summer of 2011 after Hughes had left the club. In 2012, he joined Queens Park Rangers' backroom staff, after Hughes was appointed manager. He and Mark Bowen were briefly joint caretaker managers after Hughes' dismissal in November 2012, Niedzwiecki joined up with Hughes at Stoke City in June 2013.