Thompson is taking over responsibility for building G2 in Europe from John Lynn, who was recently appointed chief executive of Grey Global Group, Spain.
Thompson and Spindler founded the forerunner to Joshua, Promotional Marketing, more than 20 years ago and have built it into an agency often named as the UK's leading integrated shop. It was bought by Grey in the mid 1990s and became Grey Integrated, before being merged with Grey Direct to form Joshua in 1998.
Spindler, previously the managing director, now takes on Thompson's duties as head of Joshua G2.
In his new role, Thompson will report to Joe Celia, worldwide chairman and CEO of G2.
Celia said: "Peter's proven vision and leadership skills will now play an invaluable part, on a broader level, in the continued growth and success of the G2 offer in the European market."