Paula Sherriff called on the store to take action after it was revealed customers using WHSmith outlets in hospitals were sometimes charged more for some items than if they visited a high street branch.
WHSmith contested some of the claims made but after meeting Ms Sherriff has confirmed it will ensure promotions are the same across its outlets in the future.
Other major chains have also faced claims that they are charging customers more in locations such as hospitals.
Ms Sherriff said: “This is a welcome first step for NHS patients, visitors and staff, who were frankly being exploited to boost the bottom line of some of Britain’s biggest businesses while the government looks the other way.
“It’s appalling that ministers have refused to act, but I will be meeting other retailers soon and I hope they will follow this lead.”
In a statement, WHSmith said: “The vast majority of our products are the same price in both our Hospital and High Street stores.
“This includes all newspapers, magazines, books and stationery. For food and drinks, where prices may be different, they are very similar and often slightly cheaper in Hospitals reflecting the different sales mix and the resulting promotional approach which is designed to best deliver value to all of our customers, despite the different cost structures.
“Going forward, our High Street stationery promotions will always be available in our hospital stores and we will continue to monitor average selling prices across all ranges to ensure they remain aligned.”