Kim Little’s fourth-minute penalty opened the scoring for the Gunners after Easther Mayi Kith brought down Katie McCabe in the area.
Frida Maanum added a second just before half-time and created Arsenal’s third when her cross was turned in by Reading defender Emma Mukandi two minutes after the restart.
Victory left Arsenal three points behind Manchester City in third place, having played one game fewer and boasting a superior goal difference.
“I think we have improved [and] developed, today was some of the best attacking football I have seen from us,” Arsenal boss Jonas Eidevall told Sky Sports.
The Gunners beat Chelsea in the League Cup final last weekend, before dispatching Liverpool in midweek.
“The [League Cup final] was the catalyst,” added Eidevall.
Maanum deservedly doubled Arsenal’s lead, slotting home Caitlin Foord’s cut-back, before the Norwegian’s cross was poked into her own net by Reading defender Mukandi.
England captain Williamson found the net for the first time in more than a year for the Gunners after Stina Blackstenius’ initial shot was saved.
Blackstenius should have added a fifth goal in the closing stages, rounding Moloney but sending her finish wide from a tight angle.
Reading are the only side without an away point in the WSL so far this season, losing all eight of their games on the road while conceding 24 goals.
They face Chelsea next on Sunday in the FA Cup quarter-finals, while Arsenal take on Bayern Munich in the first leg of their Champions League last-eight tie on Tuesday, 21 March.