Cook's previous highest score this season was 31
England opener Alastair Cook and Jaik Mickleburgh both hit centuries as Essex had the better of the opening day of their clash with Northants.
Cook (155) and Mickleburgh (112) shared 258 for the second wicket as Essex seemingly put their early-season batting woes behind them.
But Cook fell as Northants hit back with the second new ball to reduce Essex from 326-2 to 345-6 at stumps.
Ravi Bopara (31) was another victim as he again failed to make a big score.
He was trapped leg before by Chaminda Vaas (3-53) as he and Jack Brooks both struck twice in the final hour of the day to lead the Northants fightback.
Bopara has now made only 87 runs in five Championship innings this season, doing little to enhance his claims to fill the vacancy in England's middle order left by Paul Collingwood's retirement from Test cricket.
In contrast it was a welcome return to form for Ashes hero Cook, who had
scored six and 31 against Kent
19 and 25 at Middlesex
as relegated Essex lost their opening two first-class games of the season.
He was joined by Mickleburgh early on after Vaas trapped Billy Godleman leg before, but the pair soon began dismantling the home bowlers.
Cook brought up his 32nd first-class century off 144 balls, while Mickleburgh, who was dropped by Northants captain Andrew Hall at slip of the penultimate ball before tea, completed his second career century off 188 deliveries.
Lee Daggett ended the partnership by removing Mickleburgh's off stump, with Northants going on to take another four wickets for 11 runs before the close.
DAY ONE REACTION
BBC Essex's Nick Gledhill:
"Had the Essex batsmen visited a cricketing doctor after being bowled out twice in a day at Lord's the prescription would have been to win the toss under cloudless Northampton skies and get their confidence back.
"For most of the day they were on course for a full recovery thanks to Cook and Mickleburgh's superb partnership.
"Cook showed that he was not in the mood to miss such an opportunity as he accumulated relentlessly, making Northants pay for missing a hard chance just after lunch.
"Mickleburgh has enjoyed the April sunshine before - his career best 174 coming in similar conditions at Durham. Here he took his time, making sure he saw off the new ball threat to take advantage in the afternoon."