Sullivan was the chief content officer at Third Door Media, and co-founded Search Engine Land, an industry publication that covers news and information about search engines, search marketing, SEO and SEM topics. Third Door Media also produces Marketing Land, a sister website that covers broader digital marketing topics including social media, display advertising, email marketing, analytics, mobile, and marketing technology. Search Engine Land and Marketing Land are owned by Third Door Media, of which Danny Sullivan was partner and chief content officer.