My answer to How does a civilian get security clearance?
Answer by Ken Larson:
The US Government security clearance process is based on two major principles:
1. A "Need to Know"
2. A requirement to perform a job on a specific government contract containing classified work.
Initial clearances are obtained by developing a business relationship with an agency or a prime contractor performing classified work and then getting assigned to a contract effort which is classified. Thereafter, clearances at certain lower levels can be carried by employees or a company to other contracts at the same level of clearance requirement or even to a different agency within certain time constraints. At higher levels of clearances every employee is "Read On" and "Read Off" each and every contract on a singular basis after extensive background checks and in certain instances a polygraph. The process can involve lifetime nondisclosure commitments by an individual.
So you need a sponsor, a prime contractor company or an agency that will process your clearance based on your need to know classified information to participate in classified government contracts. Naturally they must be convinced via your marketing campaign that you can add value to the program involved. If it seems like a "Chicken and Egg" process – it is; but every business working in the venue has to go through the gates discussed in this article and maintain a vigilant program of compliance with associated regulations.