Mr Fenby had been editor for 18 months since the Scott Trust, owners of the Guardian, bought the Observer from Lonrho.
Mr Preston, the longest serving editor in national newspapers, was yesterday named as the editors' editor for the second year running in a poll of British newspaper chiefs.
He now takes on the role as editor-in-chief of both titles. The trust's announcement of the changes stated simply that Mr Fenby had resigned "by mutual agreement".
Staff at both the titles - housed in Farringdon Road, central London - learnt of the changes when a notice was pinned up.
Mr Preston gathered staff at the Observer last night and told them he wanted the paper "to feel loved and cherished" and that this was to be achieved by "motoring forward".
When the trust bought the Observer in 1993 its sales, then falling, stood at just over 500,700. January's figures put sales at 471,000. Regardless, Mr Preston insisted to staff that "tremendous strides" had been made.
The company's official line was that the changes would "maximise the use of editorial talent and resources available to the two newspapers" and that both would retain separate identities.
The Guardian's daily sales are 388,000 compared with 411,000 when the trust added the Sunday title.
Although the Guardian had not reduced its cover price in the current newspaper price war, it has spent heavily in marketing and advertising to retain readers.
A Farringdon Road source denied there were plans for a seven-day operation, pooling the resources of both titles.
In his official statement, Mr Preston said he was pleased to have "this new opportunity" at the Guardian but was sorry Mr Fenby had left.
Although Observer staff had been anticipating a change of regime, the departure of Mr Fenby, according to a number of staff "was still a surprise".
Many said he had perhaps concentrated too much on daily detail at the expense of providing a vision for paper's future.
Under Mr Fenby, the Observer broke the story about the Government's secret contacts with the IRA before No 10
was prepared to admit the links that led to the Downing Street Declaration in December 1993.Reuse content