An external diagonal move is when you move into a higher level or broader role in a different organisation: for example if a CFO becomes the CEO of a different organisation. They are rare. Most career moves are either vertical within the same organisation or horizontal between organisations.
External diagonal career moves are rare because you need to convince someone you don’t know that you are a better fit than someone who has already proven their effectiveness at a more senior level. For example, if you are the Director of Marketing in a mid-sized enterprise and want to become the COO or CEO in a different company of about the same size, it’s difficult to prove that you have what it takes to run a whole company without direct evidence. They want more than an assertion of your capabilities.
In addition, you are competing with people who have experience at the higher level. Because most employers are risk averse, they prefer people with a track record of accomplishment at that level rather than someone looking for a bigger job.
Employers and executive search consultants focus on the experience and the track record you have had rather than your potential. Employers are also likely to prefer people with experience in their industry. Unless you have special experience that’s rare and in high demand or a record of unparalleled success that’s hard to match, it’s difficult to convince someone to consider you ahead of people with proven records at the level to which they want to hire.
Successfully making an external diagonal move is more likely if you are recommended by someone who is credible with and trusted by the hiring executive. For example, a Group Financial Controller became the Chief Financial Officer of a new subsidiary of a different company because the COO of the holding company was once that person’s boss and persuaded the CEO of the subsidiary to consider him. His inexperience at CFO level was discounted because the COO knew him well. Had he not received such a personal endorsement from the COO, the CEO would have not considered him.