Johnson has been at Apple since 2005. Prior to joining Johnson was senior vice president, global brand and communications at Hewlett-Packard. She has also held senior marcoms roles at Netscape and IBM.
According to the report in the Wall Street Journal's digital blog Johnson is leaving before the summer to co-found a marketing communications firm with former Facebook PR executive Brandee Barker.
TBWA\Media Arts Lab counts Apple as a global client for creative, with MG OMD handling the UK media account. Apple spent almost £16m on media in the UK in 2009.
Apple declined to comment.
Arch rival Microsoft has also recently found itself on the wrong side of departing marketing execs, with the news earlier this week that Microsoft's global marketing chief Mich Mathews, who oversees a budget of more than $1bn, is to retire after the summer.
Last Friday (25 March) Apple launched its long-awaited iPad 2 in the UK.