Like England hero and his County captain Andrew Strauss, Koenig is South African-born and the highest score of his career and 13th century formed the backbone of Middlesex's total of 323 for five on the first day of their county championship clash.
His first century in two years came off 311 deliveries at Old Trafford as the rest of his side failed to really get going.
A former Middlesex hero Mark Ramprakash was also among the runs as he scored an unbeaten century for former County champions Surrey.
Jonathan Batty gave him excellent support as he too carved out an unbeaten century which was most welcome after they had slumped to 1 for two.
However Ramprakash, never the one to count on in his England days to pull off an escape act, stepped up to the plate putting on a stand of 90 with Rikki Clarke, who made 44 before being bowled by Martin Saggers just before lunch.
Ramprakash finished on 123 and Batty made 120, including 21 fours, in just 182 deliveries as Surrey posted a commanding total of 306 for three and perhaps setting themselves up for their first victory of the campaign.
Another former England failure Graeme Hick did what he has always done best in beating up average County championship bowling attacks as the Zimbabwean-born batsman struck 158 for Worcestershire against Warwickshire.
Hick carried on his rich vein of form - his last two innings were double-centuries - as he hit his 125th century of his career and was only rmeoved through a run out.
His innings, including one six and 24 fours, from 263 deliveries as Worcestershire saw his County to an imposing 373 for eight.
Champions Sussex got off to a good start in eking out the Northamptonshire top order before letting them off the hook and captain James Sales took full advantage to hit an unbeaten 153 to finish on a respectable 315 for five.
Sussex reduced the visitors to 53 for three with Robin Martin-Jenkins removing Tom Huggins (7) and Martin van Jaarsveld (27) after England international James Kirtley had dismissed opener Tim Roberts for 10.
However, Sales and Usman Afzaal enjoyed a stand of 181 before Afzaal was caught by former Zimbabwean international Murray Goodwin after making 69.