In her new role, Conklin will report to editor Louise Chunn, another former deputy editor of Elle.
Both titles remain locked in a fierce battle for the young women's market, although their combined sales fall short of Conde Nast's mammoth 537,400-selling Glamour.
Butt, who was on IPC's InStyle for less than a year, resigned in order to launch and edit independent publisher Sibella's debut title, an as-yet-unnamed women's glossy (PRWeek, 13 June). The lifestyle and travel magazine comes out next month.
Conklin, who joins InStyle on 21 July, told PRWeek: 'I'm particularly suited to celebrity journalism. Being American, I have always loved InStyle.'
Before joining Elle, which was then owned by Emap Elan, Conklin was editor of the same publisher's Celebrity Looks. 'That was a celebrity health and beauty magazine for girls. This is a grown-up version of that,' she said.
Like Butt, who worked as features editor on Loaded and Heat, Conklin's background is in features and she worked to increase the real-life story content at Elle.
Conklin had been acting editor of Elle following the departure of Fiona McIntosh, but was passed over for the top job when Sarah Bailey - another internal candidate - was preferred in March 2002.
Conklin has also been features editor and assistant editor of New Woman.
She came to the UK from New York in 1994 and worked as a trainee reporter at the Newcastle Journal. In 1996 she became features writer, then features editor, of Take a Break.
No one at InStyle was available for comment. The title came into the IPC stable when its owner, AOL/Time Warner, took over IPC in 2002. Elle was taken over by Hachette Filipacchi last year.
Conklin joined Elle when then deputy editor Rachel Loos left to become acting editor at Elle Decoration.