McKenna replaces Iain Carmichael, who is to head Audi's fleet sales department.
Seat is seeking a replacement for McKenna.
Recent Seat advertising has included a TV cam-paign for the Altea, which launched late last year and featured tennis legend John McEnroe.
However, Seat's promotional activity has not translated into sales. The car marque sold 28,431 cars in the UK last year, a fall of 14.4% on 2004, according to the Society of Motor Manufacturers and Traders.