"For many years portfolio careers were predominantly the domain of the 45 to 50-plus age group, who may no longer be in the corporate world. The other group that was involved in portfolios were working mums juggling the demands of family and a professional career," he says.
The past few decades have seen a massive change in the way in which we engage with working life. Lack of job security and the need for resilience and flexibility in the employment market has seen portfolio careers increase in popularity.
"One of the reasons for the increase in portfolio careers is the nature and changes in the way that we regard employment," Johnson says. "We all know that there are no jobs for life and, in fact, now the average person will do 10 to 12 jobs in a lifetime, hence employability rather than employment is what matters."
Portfolio careers are no longer the preserve of the middle-aged or working mothers. The freedom and diversity offered by portfolio careers means that they have found favour with younger generations who are keen to explore many different areas of working life. "Younger people (typically Gen-Y) are looking for roles where they can use their skills more broadly, work with colleagues and friends who they want to socialise with, and are also looking for something more out of the job, more than the salary," he says.
While the idea of working across a range of jobs may sound rather taxing, Johnson says portfolio careers have some very positive characteristics.
"The key benefits of a portfolio career are typically around flexibility, being able to choose the type of work you want to do and the people you work with," he says.
"There are also benefits to be had in using skills or working with people you wouldn't ordinarily get a chance to work with."
He says that in a changing employment market, having skills that can be transported to a range of different organisations can be extremely valuable. But he says people wanting to take on a portfolio career need to be flexible and resilient, and that there is a level of risk inherent in the work.
"The biggest pitfall tends to be around the degree of risk that you are comfortable with and also how structured and organised you are about your time. You need to be organised so that you can focus on acquiring new work as well as doing the work that is existing."
While some may feel that having fingers in many pies (so to speak) could mean a diminution of key skills in a damaging way, Johnson disagrees.
"I believe it's quite to the contrary," he says. "In a world where employability will be the most dominant factor over the next 20-30 years, building a broad set of skills ... is a real strength." In fact, he cites an article that appeared in the Economist in January last year that said "careers" as such will disappear in the next 15 years. "I think portfolio careers will increase in popularity as we seek more meaningful work and roles emerge we can't even conceive today.
He says that he wouldn't be surprised to see more and more people taking on this style of career.
"Having watched the emergence of portfolio careers over the last 20 years, I think we will see an increasing uptake in this. It will coincide with the notion that we will no longer 'retire' from a job. Today we are much more likely to shift through phases of paid and unpaid work, as we live longer and still need to earn income as well as staying mentally active."
Johnson has words of advice for people thinking of diversifying their careers. He recommends asking key questions around motivation for moving into such a career, then to work out what interests you, and try to gain an understanding of how comfortable you would be in the face of work uncertainty.
"From this point you need to work out where you will get your baseline income from; a regular part-time job will be able to provide this. Talk to other people who have moved into this kind of work, and create a firm plan that you will stick to."
He also says it's important to conceptualise a portfolio career as a career, rather than as a stop-gap while you work out other things to do.
"It's important that you enjoy what you do, there are some real benefits to such a career.
"And if you give it a go and it doesn't work out, at least you will have the satisfaction of knowing that you tried."