The Mariners looked the better side and had the best chance of the first-half in the 40th minute but Williams' powerful left-footer rattled off the upright.
Hyderabad FC maintained their unbeaten run in the Indian Super League
There was some drama at the end as ATK forced a series of corners but a goal did not materialise.
Valskis scored in the 30th and 66th minute while Roy Krishna pulled one back for ATKMB in the 80th minute.
Bengaluru FC played their hearts out to score an equaliser but looked clueless against an organised ATKMB defence
Aizawl FC booked a berth in the summit clash of the Federation Cup football tournament in their maiden appearance after they drew with Sporting Clube de Goa 2-2 in the second leg semi-finals, in Margao, on Sunday.
As in their last match against Kerala Blasters, ATKMB struggled to get going initially, while SCEB made some fine attacking moves but lacked the finishing touches.
FC Goa returned the favour just four minutes later when Seriton Fernandes and Alberto Noguera played a quick one-two, before sending the former free on the right flank.
Carl McHugh scored ATK's 100th goal of the ISL in only the sixth minute of the game but the home side, egged on by a vociferous crowd of more than 35,000, hit back with two goals from their new signing Ogbeche to begin their campaign on a winning note.
But the hosts did have their equaliser in the 54th minute from another corner. Rene Mihelic whipped one into the far post where Rana Gharami managed to head it back across the box. Kotal, this time around, directed it home from close-range.
A summary of sports events and sports persons, who made news on Saturday
In a first for ATK Mohun Bagan this season, two first-half goals signalled the downfall for a beleaguered Bengaluru FC
