Hennessy is one of the largest stockholders in the new company and will serve on the board of directors of Chatham, according to the letter.
"I have a strong belief in the Chatham strategy and management team and have shown my personal commitment by investing heavily in Chatham," he said in the letter.
The management team of Hennessy Products will not change and Hennessy will continue to serve as chief executive officer and president of the company, he said in the letter.
In a phone interview from Dallas on Monday, Hennessy said he plans to release more details of the merger later this week.
The company designs and manufactures stainless steel and aluminum expandable environmental outdoor enclosures and outdoor environmental cabinets that hold electronic equipment.
"I think it's going to be a good deal all the way around for Franklin County and the local economy," said Michael Ross, president of Franklin County Area Development Corporation.
Hennessy is exploring the possibility of building a larger plant in Chambersburg to accommodate the growing business, Ross said.
The company has grown from 85 employees in 1994 to about 200 now, with the expectation of doubling the work force in the next five years, Ross said.
"Mike (Hennessy) has taken steps to protect the company and provide opportunities for its growth," Ross said, adding that Hennessy, a native of Chambersburg, is committed to keeping the business local.
One of the reasons behind Hennessy's decision to merge his company is because the market's rapid growth is causing customers to change the way they do business with suppliers, he said in the letter.
Customers are focusing on reducing their supplier base and moving toward partnering with only a few companies who are capable of supplying their numerous factories around the world, Hennessy said.
"It became clear to me that Hennessy Products Inc. had to develop an international presence, grow in size quickly and have the financial management and manufacturing resources necessary to supply our key customers, as well as the new customers, in our dynamic marketplace," Hennessy said.
The new company's plan is to grow to more than $1 billion in sales within five years in part by acquiring overseas companies, according to the letter.
Hennessy was founded in 1921 by James J. Hennessy Sr., who started out manufacturing products for the railroad industry.
The company entered into the telecommunications industry in the early 1980s when it began making outdoor cabinets for traffic signals, telephone and cable television controls.