William MacKenzie and John Turner both became Prime Ministers without holding seats in the House and had to watch from the visitors gallery until a bi-election got them a seat (Turner's government fell but in the ensuing election Turner did get a seat).
Brian Miulroney was elected leader of Conservatives and this leader of oppposotion in 1983 and only later 1983 won a seat in bi-election (or is that an LGBTQ+ election nowawdays? 🙂 And won big in proper 1984 election.