Hosts Northamptonshire edged the first day of the LV= County Championship Division One encounter after bowling Yorkshire out for just 136 before reaching 191-7 at the close, but day two undoubtedly belonged to the visitors thanks to Lees and Lyth.
After Yorkshire had wrapped up the home side’s first innings for 251 in the morning session, Lees and Lyth put on an unbroken 237-run stand - the highest opening partnership anywhere in the championship this summer - to leave the White Rose county holding a lead of 122 at stumps.
Lyth made 116 with 17 fours while 21-year-old Lees struck 13 boundaries in his 105, the duo notching their 10th and fifth first-class centuries respectively.
Lees did have a life when on 78, dropped at mid-wicket, but otherwise the two left-handers were largely untroubled as they put the Northamptonshire attack to the sword.
The double-century opening stand completed a fine day for Yorkshire, which began with them taking the final three Northamptonshire wickets for the addition of 60 runs.
Ian Butler could only add one run to his overnight 13 before he was snared by Steven Patterson, Liam Plunkett taking the catch at short extra cover, but Ben Duckett and Maurice Chambers then frustrated Yorkshire with a 42-run stand for the ninth wicket.
Teenage wicketkeeper Duckett recorded only his third half-century during the stand but he fell soon after, bowled by Adil Rashid, and the innings was wrapped up when Rashid had Chambers caught by Lees at mid-on.
That left winless Northamptonshire with a first-innings lead of 115 which, considering Yorkshire’s problems with the bat on day one, looked a significant advantage.
However, home-grown duo Lyth and Lees, who between them made just 15 first time around, joined forces in a superb stand which comfortably passed the previous best opening partnership of 181 between Middlesex duo Chris Rogers and Sam Robson - made against Yorkshire in April - to leave their side in a strong position at the midway point of the match.
Read Chris Waters’s report from Wantage Road in tomorrow’s Sports Monday ONLY in The Yorkshire Post