Scoring his first goal for Auckland in his 58th match, midfielder Howieson's 60th-minute volley took a deflection on the way past wrong-footed goalkeeper Harrison Devenish-Meares, sending a sold-out crowd at Mount Smart Stadium into ecstasy.
"I'm so stoked. This was our goal from the start of the season. And we're here and we've done it," said Auckland forward Jesse Randall.
"I'm so proud of all the boys and the staff, we've put so much work into this."
Auckland joined a select group of cross-border winners in world football, emulating AS Monaco, who won eight titles in France's Ligue 1 and Northern Ireland's Derry City, twice league winners in the Republic of Ireland.
Sydney had scraped into the Grand Final via a penalty shootout following a last-gasp equaliser in the semi-final against Newcastle Jets but there was no late rally in Auckland as their bid for a record-extending sixth A-League title ended in disappointment.
Sydney captain Rhyan Grant had a late chance to equalise after latching onto a pinpoint cross from English winger Joe Lolley on 83 minutes. But he sent his glancing header just over the bar.
Only moments before, Auckland forward Jesse Randall missed a chance to bury Sydney when he blazed over from short range and with the goal at his mercy.
It was the most clear-cut chance of a mostly cagey affair in a match-up of the league's stingiest sides.
Defence held sway completely from the first whistle to beyond the interval despite the theatrical efforts of Auckland's Sam Cosgrove. The tall striker went down twice in the area at the start of each half under close attention but referee Alex King waved away his pleas for a penalty.
Auckland coach Steve Corica claimed his third A-League title as coach against his old team, having delivered the trophy for Sydney back-to-back in 2019-20. As a player, he also scored the winner for them in the inaugural 2006 Grand Final against Central Coast Mariners.
