Agnew's goal in the fifth minute has put her side in a strong position heading into the second leg at the same venue on Thursday. With a 1-0 lead, they are on the brink of advancing to the group stages of the competition and potentially joining the big names in the tournament.
Both ties are being played in Scotland, with Vorskla Poltava unable to play in Ukraine due to the war.
Celtic boss Elena Sadiku will be looking for plenty of improvement after her side were somewhat fortunate to escape from a torrid first period with their lead intact.
Caitlyn Hayes saw a shot punched over the bar by Vorskla 'keeper Kateryna Samson and Lucy Ashworth-Clifford also came close but the Ukrainian champions provided plenty of threat up front.
TrendingAnd the visitors should have levelled three minutes before the break when striker Iryna Koriash missed a golden chance with the Celtic defence all at sea.
Celtic looked stronger in the second half but once again almost paid for failing to create clear-cut chances as Kelsey Daugherty was forced to pull off a fine double save to confirm their win.
The group-stage draw will take place on Friday September 27, with the subsequent fixtures scheduled to be played on the following dates:
Champions path
SKN St Polten (Austria) vs ZNK Mura (Slovenia)
Benfica (Portugal) vs Hammarby IF (Sweden)
ZNK Osijek (Croatia) vs FC Twente (Netherlands)
Galatasaray (Turkey) vs Slavia Prague (Czech Republic)
Roma (Italy) vs Servette (Switzerland)
Anderlecht (Belgium) vs Valerenga (Norway)
FC Vorskla Poltava (Ukraine) vs Celtic (Scotland)
League path
Sporting CP (Portugal) vs Real Madrid (Spain)
Juventus (Italy) vs Paris Saint-Germain (France)
Paris FC (France) vs Manchester City (England)
Fiorentina (Italy) vs Wolfsburg (Germany)
BK Hacken FF (Sweden) vs Arsenal (England)