His remarks came as it was disclosed Great British Bake Off judge Prue Leith will advise a Government review into hospital food following the deaths of six people due to a listeria outbreak.
A "root and branch" review launched by the Department of Health and Social Care on Friday will examine whether the number of hospitals catering in-house can be increased.
Mr Johnson was speaking during a visit to Torbay Hospital in Devon, where he met catering and hospital staff and patients.
The Prime Minister said: "We get too many complaints from patients about the quality of the food and I think it does affect their experience when they are in hospital.
"And sometimes it can be something as simple as not having hot toast, and having toast actually made on the wards, so one thing you want to deliver is hot buttered toast for the patients of this country."
Health Secretary Matt Hancock called for the comprehensive review in June after six people died after contracting listeria from pre-packaged sandwiches and salads either purchased on site or given out by hospital staff.
The review will also consider whether kitchen facilities can be changed to bring more chefs into hospitals as well as using less frozen food and sourcing local fresh produce where possible.
Mr Johnson said: "The NHS does an amazing job, and they produce about 140 million meals a year. Some, of course, most patients, have a fantastic experience, the quality of the meals is too variable.
"So what we want to do is drive up standards across the board, and so we are having a review by Philip Shelley. He's going to be assisted from a culinary point of view by Prue Leith."
Mr Johnson added: "Some patients don't actually eat in hospital when they really do need to, because they do not feel the food is appetising enough, particularly elderly patients."
Celebrity cook Prue Leith has previously criticised the standard of hospital meals and called for NHS trusts to make appealing and nutritious food on-site for patients.
She said: "Millions of pounds are wasted in hospitals with food ending up in the bin, unpalatable food being the main complaint.
"I'm delighted that, at long last, Downing Street and the Department of Health have decided to do something about it.
"A hospital meal should be a small highlight, a little pleasure and comfort, and it should help, not hinder, the patient's recovery."
The review will examine the quality of the estimated 140 million meals provided to patients each year, as well as staff meals.
The review was announced as a former health chief blamed "systematic failures" in public health for 17 deaths linked to separate outbreaks of listeria and streptococcus earlier this year.
In July, NHS Mid Essex Clinical Commissioning Group confirmed 13 people had died in an outbreak of invasive Group A streptococcus.
Writing in the Journal Of The Royal Society Of Medicine on Friday, Professor John Ashton, former north-west regional director of public health, warned that years of austerity and cuts to local authority budgets had stripped the ability of environmental health departments to keep up with threats.