Having returned to the fold his rise within the game was rapid. Neilston Juniors quickly stepped in to add Peter to their ranks, and his displays for the Renfrewshire side soon had the senior scouts out in force to watch his fast-developing talent. Alex Ferguson was manager of St Mirren at the time and it was he who won the day and landed Peter's signature in 1978. Getting Weir to Love Street proved to be one of Fergie's last acts in the St Mirren managerial hot seat before his acrimonious departure from Paisley, and for a few days young Weir wondered if he head done the right wing.
New boss Jim Clunie was quick to recognise Weir's big time potential and after only four weeks as a part-timer Peter was signed on a full time contract and was pitched into the Paisley Saints first team. In his first season Weir quickly established himself in a young St Mirren side that featured among others Tony Fitzpatrick, Frank McGarvey and Billy Stark. The following season 1979-80 Peter became an influential member of a new look side including Frank McDougall and Jimmy Bone, and he had a 2-0 loss to the Dons in late April been reversed then St Mirren may have improved on a fine third place finish in the Premier league behind Aberdeen and Celtic.
Weir's performances for the Buddies had by this time come under the gaze of Scotland boss Jock Stein, and the big man included him in the Scotland side for the opening British Championship fixture against Northern Ireland in Belfast on May 16, 1980. Stein used the game, a dress rehearsal for a forthcoming World Cup qualifier against the Irish, as an excuse to experiment with the future in mind. It was Scotland's first visit to Belfast for 10 years, and for security reasons the game was played on a Friday night. The pitch was bumpy and the evening unusually sultry, and although Peter Weir and fellow debutant Gordon Strachan showed promise in flashes the overall Scottish performance was decidedly poor as they went down 1-0.
Peter retained his left flank spot for the Wales match at Hampden on May 21, which Scotland won 1-0 thanks to a superb Willie Miller goal, but Stein decided to leave the inexperienced winger out for the stern Wembley test a few days later.
During Scotland's mini-tour of Poland and Hungary in the summer, Weir was capped twice more, and incredibly the lad who had only two years experience as a senior professional was down in the history books as one of the most capped players in St Mirren's history. Although not capped the following season, Weir was watched by a bevy of top clubs, including Liverpool. But in May, 1981 it was Aberdeen who swooped for Weir's signature when he joined the Dons in a swap involving Ian Scanlon. The deal was worth an estimate £330,000 a then record transfer between two Scottish clubs.
At Pittodrie, Peter took a little time to settle, seeming to labour under his record signing tag and Alex Ferguson's stated belief that Weir was "the player to take the Dons to the next level". But he suddenly threw that aside with a dynamic two-goal performance against holders Ipswich in 1981-82 UEFA Cup first-round, second leg tie at Pittodrie. He persecuted England full back Mick Mills all evening, and was hugely instrumental in a memorable 3-1 Aberdeen win. Over the following six seasons Weir became a key man in the Dons assault on Scottish and European honours, and proved time after time he was a big occasion player. He was of course a member of the side that won the European Cup Winners Cup in Gothenburg, and his contribution against Real Madrid was immense.
Tall and well-built for a winger, Peter had an incredible ability to coast past a defender using a combination of speed, close control and a body swerve that consistently left the best of opponents tackling fresh air. His pinpoint accuracy with cross balls and from dead ball situations proved lethal, particularly in tandem with the aerial ability of Eric Black. When Weir was on song, the Dons looked unbeatable.
On March 30, 1983, Peter made the Scotland side as an Aberdeen player for the first time when he was selected in an attack orientated Scots team to face Switzerland at Hampden in a European Championship qualifier. The Scots still had slender qualification hopes, but they had to beat the Swiss to stay alive, hence the emphasis on attack. Weir created the best chance of a scoreless first half when a Richard Gough header from his accurate cross was cleared off the goal line. The Swiss created chances of their own, however, and when they took a 1-0 lead just after the break it was no more than they deserved. It was not until the visitors went 2-0 ahead that Scotland came to life. Inside the final 20 minutes Weir was prominent along with Gordon Strachan and Kenny Dalglish in a grandstand finish that saw the home side recover to gain an unlikely 2-2 draw.
But there were no further caps for Peter in the 1982-83 British Championship or on a tour of Canada and he fared little better the following season until he was recalled for another trip to Belfast on December, 1983. Again manager Jock Stein used the game as an experiment, fielding five Dons in a team who were all home Scots except Graham Souness. As has been proved in many such experiments elsewhere, "chunks" of successful club sides rarely hit it off in combination with other top players at international level, and the Scots were humbled 2-0 with Weir given little chance to shine. Unfortunately, for Peter, injuries began to creep in and from February, 1984 until August, 1986 he never enjoyed a run of much more than a dozen games without injury ruining his chances of further Scotland honours.
At club level Fergie continued to get the best out of Peter in these short spells, and he added a 1984 Scottish Cup winner's medal to the championship medal that same season. Another title gong joined his collection the next campaign, followed by another Scottish Cup winner's medal in 1986. The 1986-87 season saw Weir have his longest unbroken spell of football for three years, but a change of management at Pittodrie in November, 1986 was not to his benefit. It became apparent during 1986-87 that Peter was not part of new manger Ian Porterfield's plans, and he was transferred to Leicester City in January, 1988. After 11 months there he returned north for a second period at Love Street. At the beginning of the 1990-91 season, he moved to Ayr United where he had two full seasons before hanging up his boots in the summer of 1992. On leaving football, Peter turned his attention to a newsagents business near Hampden in Glasgow.