Security correspondent, BBC News
Earlier this week, an unusual gathering took place as a flock of rarely-seen spooks descended on central London.
Mr Evans inherits an organisation going through a lot of change
The heads of almost every Western security service gathered at a building on the banks of the River Thames for a party.
They came from across Europe and as far afield as Canada and New Zealand to bid farewell to their British counterpart, Eliza Manningham-Buller, and mark the end of her four-and-a-half years as head of MI5.
The send-off, according to sources who would rather remain anonymous, reflected the gregarious character of the outgoing director.
But the gathering formed a sharp contrast to 8 July 2005 - the low point of those four-and-a-half years - when in the same building Dame Eliza brought together all her staff from caterers to intelligence officers to tell them that what they had feared, had finally happened - an attack had got through.
But she also told them that they had a chance to make a difference.
The 7 July bombing was a watershed moment for MI5 - a moment when its darkest warnings of the threat were proved right but also when public scrutiny and criticism over its ability to stop those threats increased.
Until 1993, the name of the head of MI5 was not made public but the new head Jonathan Evans will find himself becoming a public figure, even if, like his predecessor, he avoids the TV cameras.
The director general is accountable to the home secretary and prime minister for all MI5 operations.
He will be the 16th director general since the service was founded in 1909.
Mr Evans joined the Security Service in 1980 - a time when its main job was hunting Soviet spies and, as with most recruits during those days, he spent time in this world.
But he has spent most of his career in counter-terrorism, partly in Northern Ireland where MI5's role has been controversial, but also more recently in international counter-terrorism.
He took over as head of international counter-terrorism just days before the 9/11 attacks and since 2005 had occupied the position of deputy director general, traditionally the position in which successors for the top job are groomed.
He inherits an organisation going through enormous change.
Dame Eliza Manningham-Buller was MI5 head for four-and-a-half years
By next year, MI5 will have doubled in size from where it stood in 2001.
Some argue that it should have expanded faster and earlier, but insiders say there is a limit to how quickly an organisation can adapt and still do its work properly.
MI5 has also been gingerly emerging from the shadows and into the heart of an often heated public and political debate.
Countering terrorism is inherently more public than its old work.
The public expect to be told if any intelligence on the 7 July bombers was missed or what information led to raids such as Forest Gate.
MI5 has responded by expanding its own website which includes information on the national threat level.
Ensuring public confidence will be a major challenge for Jonathan Evans, reassuring people that everything is being done to deal with the terrorist threat but at the same time responding to the concerns over intrusiveness and surveillance particularly from Muslim communities.
Many others also question the extent of the terrorist threat and the way it is portrayed by the government and security officials.
'At full stretch'
Mr Evans will also have to navigate the occasionally treacherous waters of Whitehall.
A new refocused Home Office - to which MI5 reports - is about to emerge and there is a realisation that the government's overall counter-terrorist strategy - of which MI5 forms only one part - requires an overhaul.
Getting that right and dealing with issues like radicalisation will determine just how lengthy and difficult a task Jonathan Evans will face in dealing with the terrorist threat.
In her final public speech as director last November, Dame Eliza Manningham-Buller made clear that Mr Evans will be inheriting an organisation which is at full stretch.
She warned that MI5 was watching 200 networks comprising more than 1,600 individuals - a figure which has since risen even higher.
She warned that the hard choices of prioritisation involved in the work of MI5 were always going to involve risk and that there will be little tolerance when mistakes are made.
These are warnings that Mr Evans will be more than aware of as he steps, at least partially, into the spotlight.