The Welsh Government said in May 2010 health boards should ensure key workers were in place by the end of March 2011.
But a number of cancer patients across the country told BBC Wales they have not been assigned a key worker. One patient, Nick Phillips, said it "takes months to talk to anybody".
A Welsh Government spokesman said services had made "a positive start".
In 2010, the then health minister Edwina Hart said introducing key workers meant cancer patients would know who to contact at all times should the need arise during their treatment.
But Mr Phillips, from Pontypridd, said he had not been assigned a key worker despite being diagnosed with prostate cancer in January 2016.
The 59-year-old, who runs a cancer charity from home, said: "I haven't heard of anyone around here who's had a key worker, someone you could phone up, from diagnosis, to get support and information."
Mr Phillips has undergone chemotherapy and radiotherapy, but said his cancer cannot be cured, only controlled, because it has spread.
"Having a key worker would have changed my experience. Rather than me chasing around, that key worker would be able to point me in the right direction and be able to help me. It takes months to talk to anybody," he said.
"Once you've had treatment, you're forgotten about."
Carolyn Robertson, 70, from Whitchurch, in Cardiff, has been diagnosed with three different types of cancer since 2010 and has experienced both sides of the coin.
Mrs Robertson said she was not allocated a key worker for the bowel cancer she was diagnosed with in 2010 nor the skin cancer she was diagnosed with in 2016.
But she said she was given a key worker when she was diagnosed with breast cancer in 2012.
Mrs Robertson said: "I think it's very important because, first of all, it's the comfort of knowing a name and number so you don't have to go through a switchboard and hold on to be put through.
"That person will also have the experience and knowledge to say if you need to come in and be checked.
"With Julie [my breast cancer key worker] it honestly felt out of my hands but being dealt with. It was such a relief."
Comments on the Facebook page One Voice for Wales - Campaign for Equal Access to Cancer Treatments, appear to show the experience of patients varies, even within health boards.
Two people from Swansea commented that they have non-Hodgkins lymphoma and have not been allocated a key worker.
But a breast cancer patient being treated in Swansea said she has had a key worker for the last two years.
All health boards in Wales were contacted by BBC Wales and asked whether all their cancer patients had been allocated a key worker.
Dr Graham Shortland, medical director for Cardiff and Vale health board, said they were currently providing key workers to more than 90% of patients but were aware some patients still needed support.
"If any patients do not feel they have this, we urge them to contact our Macmillan Cancer Lead Nurse who will ensure the patient is allocated a key worker and receives the appropriate support," he said.
Aneurin Bevan University Health Board said its latest audit showed 93% of its cancer patients were allocated a key worker and it was working towards 100% compliance.
A spokesman for Betsi Cadwaladr health board said it was "continuing to aim for 100% compliant".
Abertawe Bro Morgannwg, Cwm Taf, and Hywel Dda health boards said a Freedom of Information request would need to be submitted to get details of key worker allocation.
A Welsh Government spokesman said: "Our updated Cancer Delivery Plan commits cancer services to providing each person affected by cancer with the name and contact details of a designated key worker.
"The 2014 cancer patient experience report showed services had made a positive start with 66% of people affected by cancer reporting being assigned a key worker, we hope to see this improve when the 2016 report is published."
Susan Morris, head of services for Macmillan Cancer Support in Wales, which carried out that survey, said: "We know not everyone with cancer has a key worker more than six years on from this commitment."
She said: "Macmillan Wales wants every person diagnosed with cancer to have access to a clinical nurse specialist who also fulfils a key worker role, and we have consistently campaigned for this over the last seven years."
Plaid Cymru's health spokesman, Rhun ap Iorwerth AM, said: "There are still far too many people diagnosed with cancer who do not have access to a key worker.
"When we highlight this, and ask why this pledge has yet to be delivered, we have been told that everyone does have a key worker, it's just that recording the data was the problem."
Angela Burns AM, the Welsh Conservatives' health spokeswoman, said: "Back then [in 2010] key workers were hailed by the health minister as 'central' to improving cancer care - maybe their view has changed.
"Either way, recovering cancer patients have the right to know when they can expect the Labour government to deliver the level of care they need and were promised."
UKIP health spokeswoman Caroline Jones AM said: "As a cancer survivor myself, I know first hand the minefield that is cancer treatment and having someone who can support you and guide you is vital.
"The Welsh Government must guarantee that health boards will provide a key worker to every cancer patient in Wales."
Some cancer patients in Wales say they have still not been allocated a key worker to co-ordinate their care.