Mahlangu played for the Swedish Double winners as they defeated Welsh side TNS 3-0 to secure their passage to the next round by the same scoreline on aggregate.
Helsingborg will now face Polish side Śląsk Wroclaw and, if they win, will be into the play-offs. Success there will see them enter the lucrative group stages.
It has not been such a good week for two other South African-born players in the competition though.
Ricardo Nunes and his Slovakian side Zilina were defeated 2-0 by Israeli outfit Ironi Kiryat Shmona to drop out of the competition 2-1 on aggregate.
Nunes missed a penalty late on in the first leg which could have out a different complexion on the tie, and the Johannesburg-born fullback will now play no further part in European competition this season.
And young goalkeeper Reyaad Pieterse had to watch from the bench as his Republic of Ireland club Shamrock Rovers lost 2-1 to Lithuanian side Ekranas to drop out of the competition by the same scoreline on aggregate.
There is one South African entering the competition in the third round as Kamohelo Mokotjo and his Dutch side Feyenoord face a tough tie with Ukranian outfit Dinamo Kiev.
Bafana playmaker Thulani Serero and his Ajax Amsterdam team will start their campaign in the group stages, having won the Dutch Eredivisie last season.
The third round ties will be played over two legs on July 31/August 1 and August 7/8.
UEFA Champions League third round fixtures:
NK Maribor (SVN) v F91 Dudelange (LUX)
FC BATE Borisov (BLR) v Debreceni VSC (HUN)
CFR 1907 Cluj (ROU) v FC Slovan Liberec (CZE)
RSC Anderlecht (BEL) v FK Ekranas (LTU)
WKS Śląsk Wroclaw (POL) v Helsingsborgs IF (SWE)
FC Sheriff (MDA) v GNK Dinamo Zagreb (CRO)
Celtic FC (SCO) v HJK Helsinki (FIN)
Molde FK (NOR) v FC Basel 1893 (SUI)
Hapoel Kiryat Shmona FC (ISR) v Neftçi PFK (AZE)
AEL Limassol FC (CYP) v FK Partizan (SRB)
Fenerbahçe SK (TUR) v FC Vaslui (ROU)
Motherwell FC (SCO) v Panathinaikos FC (GRE)
FC København (DEN) v Club Brugge KV (BEL)
FC Dynamo Kyiv (UKR) v Feyenoord (NED)