After producing a 40-point game against the
West Adelaide Bearcats in the local Adelaide competition in early 1985, Ninnis was invited to train with National Basketball League team the Adelaide 36ers by 36ers coach Ken Cole in 1985, training against NBL stars such as
Al Green,
Darryl Pearce,
Mike McKay, Ray Wood,
Bill Jones, Peter Ali and
Mark Davis. Ninnis made his NBL debut on 12 July 1986 during Round 10 of the
1986 NBL season against the
Perth Wildcats at the home of the 36ers, the
Apollo Stadium. He would go on to play 13 more regular season games for the team which compiled a league best 24–2 record during the regular season, earning the team the nickname "The Invincibles". The
1986 Adelaide 36ers would go on to win their first
NBL Grand Final in
1986, defeating the
defending champions the
Brisbane Bullets 2–1 in the Grand Final series. Ninnis only got to play in the final 30 seconds of Game 3 during the series which the 36ers won 113–91. 36ers coach Ken Cole was sacked after a
marijuana smoking controversy in 1986 and was replaced by former NBL import guard Gary Fox. Unfortunately for Ninnis, Fox sent him down to play with the Adelaide Buffaloes in the
SEABL in 1987 and he only played 2 NBL games during the
1987 NBL season (for a total of just 5 minutes and 18 seconds). However, it was not a total loss for Ninnis as the Buffaloes would go on to win the SEABL championship that year after defeating the
Ballarat Miners in the Grand Final. Ninnis was recalled back into the 36ers squad by Fox full-time from
1988. He would remain with the club until the end of the
1990 NBL season before making the decision that if he wanted to further his game he would need a change of scenery. He then signed with the
Brian Goorjian coached
Eastside Melbourne Spectres in
1991 NBL season and played in his second NBL Grand Final that year, though the Spectres went down to the defending champion Perth Wildcats. While playing for the 36ers, Ninnis continued to play for the
South Adelaide Panthers in the
SA State League, winning the championship in 1987 and again in 1989. The Spectres and the
Southern Melbourne Saints merged prior to the
1992 NBL season to become the South East Melbourne Magic with Ninnis signing with the new team who would be coached by Brian Goorjian. Alongside teammates including
Tony Ronaldson,
Bruce Bolden,
Robert Rose,
John Dorge and Australian Boomers head coach
Andrej Lemanis, Ninnis won his second NBL Championship after the Magic defeated cross-town rivals the
Melbourne Tigers 2–1 in the Grand Final series. During Game 1 of the series, Magic
point guard Darren Perry went down with an injury. Ninnis, normally a
shooting guard, stepped in and played the point for the remainder of the series. Ninnis returned home to play for the 36ers in
1993 and would go on to win that season's
NBL Most Improved Player award. Statistically, 1993 would be Ninnis's best year in the NBL, averaging 19.4 points, 3.1 rebounds and 4.7 assists for the season. In
1994 under the coaching of
Mike Dunlap, the 36ers reached their first NBL Grand Final since 1986, but the team would be swept 2–0 in the series by the
North Melbourne Giants. At the end of the
1995 NBL season where the 36ers were beaten by the eventual champion Perth Wildcats in the semi-finals, Ninnis was cut from the team by Dunlap who as a former
NCAA college basketball coach, had gained a reputation for favouring the younger players at the expense of the team veterans (during this period the 36ers also cut veterans
Phil Smyth (1994), Robert Rose and Mike McKay (1996). Upon returning to Adelaide in 1993, Ninnis rejoined South Adelaide in the local State League and won the championship for the third time in 1995. That year he also won the Woollacott Medal as the State League's fairest and most brilliant player. After being cut by the 36ers, Ninnis signed for the struggling
Newcastle Falcons for the
1996 NBL season. Ninnis would only spend one year with the Falcons before again returning to the 36ers in
1997 after Dunlap left the club and was replaced with Dave Claxton. Phil Smyth returned to the 36ers in
1998 as a rookie head coach and immediately turned the 36ers into a championship winning team. Alongside team captain
Brett Maher, Mark Davis (the only other survivor of the 1986 championship team),
Martin Cattalini,
Paul Rees and former
Denver Nuggets players
Darnell Mee and
Kevin Brooks, the 36ers would win their second and Ninnis's third NBL championship in 1998 after sweeping the Magic, whose own record that year had almost rivaled that of the 1986 36ers, 2–0 in the Grand Final. Ninnis won his fourth SA State League championship with South Adelaide in 1997. Ninnis retired from playing following the 1998 NBL season, having played 318 games and averaging 9.2 points, 1.7 rebounds and 2.2 assists per game. ==Coaching career==