Halesowen started on the front foot as Jamie Ashmore whipped in a cross from the right for leading goalscorer Jamie Molyneux to head inches wide after just 15 seconds. Visiting goalkeeper Ryan Schmid then dived to save at the feet of Montel Gibson in the fourth minute.
But the Yeltz mostly struggled in a disjointed first half performance and the Citizens stunned the hosts to race into a two goal lead. Jesse Walklin played a great cross field ball over the top of the home defence to find fullback Jay Rolfe on the right of the penalty area and he easily beat home goalkeeper Brendon Bunn in the 6th minute.
The Citizens carved open the home defence again after 21 minutes but Brendon Bunn raced out of his area to win the ball. George Ironton proved sharper than the home defence when he doubled Welwyn's lead after 27 minutes by reacting first to convert a rebound after Walklin's shot was saved.
The lack-lustre Yeltz found it difficult to respond although Ashley Sammons saw his 30th minute shot from a free-kick deflected away for a corner and
Sammons then fired on top of the net five minutes later. The visitors were reduced to ten men a minute before the interval when goalscorer Rolfe was dismissed for his second yellow card.
Halesowen had no answers after coming up against robust Welwyn's wall of defence after the interval. The early part of the second period became a stop-start, ill-tempered affair with few chances as the Citizens picked up three further yellow cards around the hour mark. Sammons continued to create the best home chances and his 67th minute deflected shot looped up onto the crossbar and Sammons also drove wide with time running out for the below-par hosts.
Welwyn were reduced to just nine men when Carl Mensah received a straight red card for a foul on the edge of the penalty area in the final minute but the well marshalled visitors easily saw out seven minutes of stoppage time to remain unbeaten in the league.
Halesowen Town: Brendon Bunn; Jamie Ashmore, James Bowen (George Forsyth 75), Ashley Sammons, Paul McCone (c), Andrew Parsons, Joshua Hawker (Luke Yates 53), Robbie Bunn, Montel Gibson (Jamie Lucas 53), Reece Hewitt, Jamie Molyneux. Subs Not Used: Robert Evans, Kartel O'Neill-Martin.
Welwyn Garden City: Ryan Schmid; Jay Rolfe, Jonathan Sexton, Jack Bradshaw, William McClelland, Carl Mensah, Lee Close, George Ironton (Dylan Ebengo 90), Toby Cleaver (Eusebio Da Silva 68), David Keenleyside, Jesse Walklin. Subs Not Used: Charlie Jones (GK), Charlie Payne, Gerardo Smaldone.
Referee: Jamie Conde