The young winger caught the eye throughout with his lightning pace and willingness to run at his full-back as Clive Allen's side turned possession into goals in the second half.
Lennon scored one and created one of those second half goals as we finally broke Watford's resistance having gone in level at half-time.
The only down side of the night was an injury to Mounir El Hamdaoui who appeared to turn his ankle and was stretchered off in the second half. Tom Huddlestone also appeared to pick up a knock in the second half.
We fielded a strong side with the likes of Lennon, Huddlestone, Fredi Kanoute, Stephen Kelly and Goran Bunjevcevic all in the starting XI but Watford deserve credit for their first half showing, rewarded by Francino Francis' equaliser in injury time.
Both sides started brightly before Kanoute sublimely opened the scoring after nine minutes. Jackson found himself in space on the left and floated in the perfect cross that the Mali striker despatched in style on the volley.
Davenport twice went close - again from Jackson crosses - and Ziegler and Huddlestone zipped shots just wide but Watford were far from out of it as Francis fired wide from a decent position with Davenport throwing himself in to block.
That chance sparked us into our best spell possession of the half with Huddlestone and O'Hara pulling the strings centrally and Bunjevcevic hitting his targets from the back.
Indeed, Bunjevcevic played the key pass as El Hamdaoui got himself into decent scoring positions twice in five minutes - he hit the first into the ground from Kanoute's cutback and then did well to reach Lennon's low cross but couldn't keep his effort on target at full stretch.
Things looked to be going well enough into added time but Watford then pounced on a rare mistake in possession from O'Hara. Francis nicked the ball away and shot, Fulop parried low to his right but the striker's momentum took him to the rebound and he tapped into an empty net for 1-1.
The second half started slowly before two goals in three minutes put us in the driving seat.
Lennon bagged his first goal for us with a low drive through Lee's legs from Ziegler's pass after 53 minutes before a fine finish on the volley from Davenport at the far post after Kanoute flicked on Ziegler's corner for 3-1 with 56 on the clock.
We enjoyed long spells of possession after that double as El Hamdaoui volleyed wide and saw Lee block a near-post drive but then a blow for the striker - stretchered off after appearing to turn on his ankle in the 77th minute.
That meant a late run for last season's top scorer Barnard and he was back on the scoresheet 10 minutes later, in the right place to tap in after a magical run from Lennon left defenders in his wake.
Barnard might have scored again a minute later from Jackson's pass but apart from El Hamdaoui's injury, it was an ideal start.
GUIDE TO THE ACTION
4mins: Davenport rose well to meet Jackson's corner at far post but headed wide.
8mins: Francis poor header after Devaney cross found him unmarked 12 yards out.
9mins: GOAL. Deep Jackson cross volleyed home in style back across Lee by Kanoute.
Spurs 1, Watford 0
17mins: Huddlestone flashed a 25-yarder just wide.
23mins: Davenport glanced over a free header from 10 yards from Jackson's free-kick.
26mins: Davenport inches away from Jackson's wicked inswinging corner.
27mins: Ziegler completed miscued 30-yard shot, ball came straight back to him and second attempt flew a yard over.
28mins: Osborne break down right, cross from byline found Francis who laid back for Grant to screw wide from decent position as Davenport threw himself in to block.
33mins: Bunjevcevic picked out Kanoute's run with 50-yard pass, he crossed early for supporting El Hamdaoui who hit shot into ground and wide.
35mins: Good spell of possession, lots of players involved, Bunjevcevic eventually threaded pass to El Hamdaoui, he pushed it wide to Lennon and went for return at near post, did well to get between defenders before prodding just wide.
39mins: Hand took quick free-kick, Devaney helped it on to Grant breaking through, Fulop out quickly to avert danger.
Added time: GOAL. O'Hara hustled off the ball by Grant, Fulop saved first effort but Grant followed in for tap in off rebound.
Spurs 1, Watford 1
Half-time: Spurs 1, Watford 1
53mins: GOAL. Ziegler's crossfield pass picked out Lennon, he controlled, ran at Homand and drilled low through the legs of Lee.
Spurs 2, Watford 1
56mins: El Hamdaoui pulled Fulop's clearance out of the sky, cut inside and hit shot well blocked by Griffiths.
56mins: GOAL. From corner, Kanoute flicked on Ziegler's cross and Davenport volleyed back across Lee into far corner from six yards.
Spurs 3, Watford 1
59mins: Best move of game, lovely floated pass from Huddlestone released Kelly down right, he crossed for Kanoute who headed down for El Hamdaoui to volley wide from 18 yards.
67mins: Quick Lennon break despite having shirt pulled, released El Hamdaoui on goal, low shot saved at near post by Lee.
75mins: Mahon 30-yarder just wide with Fulop at full stretch.
87mins: Lennon dribbled through four defenders, cut into the box and squared for Barnard to tap home from six yards.
Spurs 4, Watford 1
89mins: Jackson pass released Barnard on goal but Lee out sharply to block one-on-one chance.
Final score: Spurs 4, Watford 1
Spurs: Fulop, Kelly, Ziegler, Huddlestone (McKie, 71), Bunjevcevic, Davenport, Lennon, O'Hara, Kanoute, El Hamdaoui (Barnard, 78), Jackson. Subs: Ifil, Burch, Hallfredsson.
Watford: Lee, Osbourne, Sietes, Mahon (Gill, 59), Griffiths, Homand (Mariappa, 69), Devlin (Campana, 59), Hand, Grant, Francis, Devaney. Subs: De Villiers, Parkes.
Mr N Kinseley; Mr O Saliy, Mr R Sidlin. Fourth official: Mr J Parker.
Aaron Lennon opened his account as our reserves kicked-off their league campaign with a 4-1 win against Watford on Monday night.